Not applicable.
Not applicable.
1. Field of the Invention
This is an improvement in the process for separation of soap from soapstock. The process overcomes problems with an intractable emulsion that occurs between acidulated fatty acids and water by treatment of the soapstock with both an acid and a monohydric alcohol or mixture of monohydric alcohols. The reaction is unique in that it allows the quantitative recovery of fatty acids from soapstock with the addition of monohydric alcohols and stoichiometric addition of acid. Large excesses of acid are required for acidulation without the addition of monohydric alcohols. The process aqueous by-product stream is recovered by ultrafiltration, electrolysis and ion exchange chromatography to minimize process waste and recover value-added products streams from the soapstock by-product water.
2. Description of the Related Art
Crude glyceride oils such as soybean and canola oil contain free fatty acids and other impurities. Contacting the crude oil with aqueous alkali in a batch or continuous process refines such crude oil. The alkali reacts with the free fatty acids to form soap that may be separated from the oil by settling or centrifugation. The separated soap material is referred to as soapstock that is a lipid-rich byproduct of vegetable oil refining. It contains substantial amounts of glyceride, phosphoglyceride and free fatty acid, the latter as their sodium or, less commonly, potassium salts. From soybean, the predominant source of edible oil in the United States, soapstock is generated at a rate of about 6% of the volume of crude oil produced (Anonymous, Soya Bluebook Plus, Soyatech, Inc., Bar Harbor, Me., 1995, p. 262.), amounting to as much as approximately one billion pounds of soapstock annually. Its price can be as low as one-tenth that of refined vegetable oil. Where possible, recovery of value from soapstock is achieved by adding a mineral acid to the soapstock to separate lipids from the water, sodium and other contaminants.
When allowed to settle acidified soapstock separates into 3 layers. The top layer is the desired fatty acid rich lipid product. The middle layer is an emulsified material containing lipid, salts and water and the bottom layer contains primarily salt water and dissolved organic compounds. While the top layer has considerable value as a source of lipids the other layers provide little or no economic return. The emulsion layer produced from splitting soapstock can detrimentally affect the recovery of the oil product. Johansen et al. (Journal of the American Oil Chemist""s Society Vol. 73, pp. 1275-1286) extracted six soapstock samples by sequentially adding sulfuric acid then centrifuging the samples at 8,000-x gravity and measured the volume of the three layers. The emulsion layers varied from 0 to 56% of the total post acidulation sample volume. Four of the six samples produced 26 percent emulsion or greater. Their analysis further revealed that the middle emulsion layer entrained large amounts of lipids that hampered efficient separation of lipid and water layers. The difficulty of separating oil and water from acidulated soapstock also occurs in industrial separations. For example, Brister (U.S. Pat. No. 4,671,902) reacted 75,000 liters of soybean soapstock with sulfuric acid to obtain 28,000 kg of fatty sludge containing 17,000 kg of fatty acids. Brister recovered the fatty acids from the sludge by addition of antifoam and spray drying the entire sludge volume to reduce the water content to less than 3.5 percent. This method though effective required the evaporation of almost 10,000 kg of water, a process step requiring substantial amounts of energy.
Dowd (Journal of the American Oil Chemist""s Society Vol. 73 pp. 1287-1295) analyzed cottonseed soapstock and found that it contained a series of water-soluble compounds. Johansen et al. (Journal of the American Oil Chemist""s Society Vol. 73, pp. 1275-1286) reported between 2.5% and 15% dissolved solids in the acid water of acidulated soapstock. The dissolved solids certainly contained sulfuric acid and sodium sulfate as residual salts from acidulation but many potentially valuable soluble compounds were also identified. These compounds included glycerol and inositol phosphates as well as other carbohydrates. Johansen et al. further noted that potentially valuable phosphorylated carbohydrates were present in the acid water and that these compounds might be recovered by strong base ion exchange chromatography. However the presence of large amounts of sulfate ion in these preparations would certainly prohibit efficient recovery of these materials from soapstock using ion exchange chromatography.
Some methods for the production of esters of lower alcohols from soapstock have been reported (e.g. Canadian patent application 2,131,654). However, the adoption of these may be limited by their use of elevated temperatures and pressures, incomplete esterification of all fatty acids in the starting material, and/or relatively long incubation times. Haas et al (Haas, M. J., and K. M. Scott, Combined Nonenzymatic-Enzymatic Method for the Synthesis of Simple Alkyl Fatty Acid Esters from Soapstock, J. Am. Oil Chem. Soc. 73:1393-1401 (1996)) described a two-step method for the production of biodiesel from soapstock. However, this method achieved esterification of only 81% of the total fatty acids, and involved the use of lipase catalysis. It is possible that the cost and operational requirements associated with enzymatic catalysis might retard adoption of the process.
Soapstock, a by-product of the oilseed processing industry, contains lipids, water and water-soluble organic materials. Although soapstock has a relatively low value it may be readily converted into more valuable products by addition of strong mineral acids in a process known as acidulation. The conventional process for acidulation and recovery of lipids from soapstock requires the addition of large excesses of acid at high temperatures to recover the fatty acid rich oil. In the conventional process a large volume of emulsion is formed that entrains lipids making them unavailable and the resulting emulsion has little commercial value in spite of its lipid content. The present invention discloses a unique process for total recovery of lipids from soapstock by acidulation in a solution containing a monohydric alcohol. The process occurs at lower temperatures than the conventional method and requires the addition of only enough acid to hydrolyse the soaps present in the soapstock. Once recovered, the lipids may be readily sold as acidulated soapstock or converted advantageously to alkyl esters of monohydric alcohols. Acid water produced by this extraction method has a lowered content of inorganic solutes and may be efficiently deionised by any of a number of commercial methods. The deionised by-product water may then be efficiently extracted to concentrate more valuable dissolved components.